(1) Field of the Invention
The present invention relates to traveling wave optical modulators.
(2) Related Art Statement
First, the speed mismatching of the traveling wave optical modulator will be explained. In the traveling wave electrode, the speed of the light traveling through the optical waveguide largely differs from that of electric signals (macrowaves) propagating in the electrode. Assume that the speeds of the light propagating in the crystal and that of the microwave are taken as Vo and Vm, respectively. For example, an LiNbO3 optical modulator with a planar type electrode is as follows. The LiNbO3 single crystal has a refractive index of 2.15, and the speed of the light traveling the optical waveguide is in reverse proportion to the refractive index. On the other hand, the effective refractive index of the microwave is given by a square root of a dielectric constant near a conductor. The dielectric constant of the LiNbO3 single crystal is uniaxial: 28 in a Z-axis direction and 43 in a Y-axis direction. Therefore, under consideration of an effect of air having a dielectric constant of 1, the effective refractive index of the microwaves in the LiNbO3 optical modulator which has no Si buffer layer and the thinner thickness T of the electrode film, is about 4, which is about 1.9 times as much as 2.15. Therefore, the speed of the light wave is about 1.9 times as much as that of the microwaves.
The light-modulating band width xe2x80x9cfmxe2x80x9d or the upper limit that of the modulating speed is in proportion to a reciprocal of a difference in speed between the light wave and the microwave. That is, fmxe2x88x9d1/(Voxe2x88x92Vm). In case of the LiNbO3 optical modulator as mentioned above, assuming that the electrode less is 0, the band width fm x the electrode length xe2x80x9c1xe2x80x9d has a limit of 9.2 GHz-cm. The longer the electrode, the more conspicuous is the influence of the limit of the operation speed. Therefore, it is strongly demanded to put into existence an optical modulator having a wide band range and a highly effective characteristic.
In order to solve the above problem, the present inventors reported in JP-A 10-133,159 that a traveling wave optical modulator can be successfully operated at 10 GHz or more when a thin portion is provided at a substrate of the modulator under the optical waveguide, and the thin portion is reduced to 10 xcexcm or less. In the following,the relationship between the thickness of the thin portion and the modulating band range is shown.
In an actual production of such a modulator, however, it was difficult in working to thin the substrate. Particularly, if the substrate is thinner than 10 xcexcm, the yield decreases due to cracking of the substrate. As the thickness of the substrate is reduced to thinner than 10 xcexcm, an effect of trapping the light inside the optical waveguide in a vertical direction becomes stronger, so that the field of the optical waveguide mode is deformed flat. Therefore, the mismatching increases between the optical field of the waveguide thinned portion and the optical field of the waveguide non-thinned portion or the optical fiber, thereby increasing a coupling less. In order to solve these problems, it is necessary that a velocity matching condition is found out in a modulating range of not less than 100 GHzxc2x7cm, while maintaining the thickness of the substrate at not less than 10 xcexcm.
It is an object of the present invention to enable a velocity matching between microwaves and optical waves in a modulating range of not less than 100 GHzxc2x7cm in a traveling wave optical modulator, while maintaining the thickness of the substrate at not less than 10 xcexcm, while not necessitating a reduction in thickness of a substrate down to less than 10 xcexcm.
The present invention relates to a traveling wave optical modulator comprising a substrate made of a ferrodielectric electro-optic single crystal and having a pair of opposing main planes, an optical waveguide formed on a side of one of the main planes of the substrate, and a pair of electrode films which apply a voltage for modulating a light transmitting through the optical waveguide and between which the optical waveguide is located, wherein the thickness of each of the electrode films is not less than 20 xcexcm and a width of a gap between a pair of the electrode films is not less than 25 xcexcm.
These and other objects, features and advantages of the invention will be appreciated when read in connection with the attached drawings, with the understanding that some modifications, variations and changes of the invention could be made by the skilled in the art to which the invention pertains.